An important component of disposable absorbent articles such as diapers is an absorbent core structure comprising water-swellable polymers, typically hydrogel-forming water-swellable polymers, also referred to as absorbent gelling material, AGM, or super-absorbent polymers, or SAP's. This polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness.
Especially useful water-swellable polymer materials or SAP's are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or poly-functional monomers such as N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to lightly cross-link the polymer chains thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked absorbent polymers contain a multiplicity of neutralized carboxylate groups attached to the polymer backbone. It is generally believed, that these carboxylate groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network.
In addition, the polymer particles are often treated as to form a surface cross-linked layer on the outer surface in order to improve their properties in particular for application in baby diapers.
Water-swellable (hydrogel-forming) polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen polymer particles to resist deformation under an applied stress in the absorbent article. The gel strength needs to be high enough in the absorbent article so that the particles do not deform and fill the capillary void spaces to an unacceptable degree causing so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake or the fluid distribution, i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article and leakage from the absorbent article can take place well before the water-swellable polymer particles are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article. Thus, it is important that the water-swellable polymers (when incorporated in an absorbent structure or article) maintain a high wet-porosity and have a high resistance against deformation thus yielding high permeability for fluid transport through the swollen gel bed.
Absorbent polymers with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure such as the pressure caused by the wearer, but this typically also reduces the absorbent capacity of the gel undesirably.
The inventors have found that often the surface crosslinked water-swellable polymer particles are constrained by the surface-crosslinking ‘shell’ and cannot absorb and swell sufficiently, and/or that the shell is not strong enough to withstand the stresses of swelling or the stresses associated with performance under load.
The inventors have found that the coatings or shells of the water-swellable polymers, as used in the art, including surface cross-linking ‘coatings’, break when the polymer swells significantly or that the ‘coatings’ break after having been in a swollen state for a period of time. They also have found that, as a result thereof, the coated and/or surface-crosslinked water-swellable polymers or super-absorbent material known in the art deform significantly in use thus leading to relatively low porosity and permeability of the gel bed in the wet state, especially under pressure. They have found that this could be detrimental to the optimum absorbency, liquid distribution or storage performance of such polymer materials.
Thus, the inventors have found that what is required are water-swellable materials comprising coated water-swellable polymers that have a coating that can exert a force in the wet state and that does substantially not rupture when the polymers swell in body liquid under typical in-use conditions. In the context of this invention, the inventors have found that as a good representative for body liquids such as urine, a 0.9% sodium chloride by weight in water solution, further called “0.9% saline” can be used. Therefore the inventors have found that it is required to have coated water-swellable materials where the coating does substantially not rupture when the materials swell in 0.9% saline.
The inventors have now developed a process for making a new water-swellable material and a new water-swellable material, comprising water-swellable polymers that are coated with a coating agent, which is extensible, e.g. elastomeric coatings which are elastically extensible, so that when the internal core of the hydrogel polymers swells, the coating can extend and remains substantially intact, i.e. without breaking.
Moreover, they have found that not all extensible materials are suitable in every application as coating agents, because some materials have a good extensibility (elongation) when in a dry state, but not in a wet state. Thus, they have found that it is important to provide the water-swellable polymers with a coating agent that is wet-extensible.
The inventors further have found that the process of applying and subsequently treating the coating agents may be important in order to impart high extensibility (elongation) in the wet state.